1. Field of the Invention
The present invention generally relates to the fabrication and design of semiconductor chips and integrated circuits, more specifically to a method of performing buffer insertion to manage timing and electrical requirements in an integrated circuit design.
2. Description of the Related Art
Integrated circuits are used for a wide variety of electronic applications, from simple devices such as wristwatches, to the most complex computer systems. A microelectronic integrated circuit (IC) chip can generally be thought of as a collection of logic cells with electrical interconnections between the cells, formed on a semiconductor substrate (e.g., silicon). An IC may include a very large number of cells and require complicated connections between the cells. A cell is a group of one or more circuit elements such as transistors, capacitors, resistors, inductors, and other basic circuit elements grouped to perform a logic function. Cell types include, for example, core cells, scan cells and input/output (I/O) cells. Each of the cells of an IC may have one or more pins, each of which in turn may be connected to one or more other pins of the IC by wires. The wires connecting the pins of the IC are also formed on the surface of the chip. For more complex designs, there are typically at least four distinct layers of conducting media available for routing, such as a polysilicon layer and three metal layers (metal-1, metal-2, and metal-3). The polysilicon layer, metal-1, metal-2, and metal-3 are all used for vertical and/or horizontal routing.
An IC chip is fabricated by first conceiving the logical circuit description, and then converting that logical description into a physical description, or geometric layout. This process is usually carried out using a “netlist,” which is a record of all of the nets, or interconnections, between the cell pins. A layout typically consists of a set of planar geometric shapes in several layers. The layout is then checked to ensure that it meets all of the design requirements, particularly timing requirements. The result is a set of design files known as an intermediate form that describes the layout. The design files are then converted into pattern generator files that are used to produce patterns called masks by an optical or electron beam pattern generator. During fabrication, these masks are used to pattern a silicon wafer using a sequence of photolithographic steps. The component formation requires very exacting details about geometric patterns and separation between them. The process of converting the specifications of an electrical circuit into a layout is called the physical design.
Cell placement in semiconductor fabrication involves a determination of where particular cells should optimally (or near-optimally) be located on the surface of a integrated circuit device. Due to the large number of components and the details required by the fabrication process for very large scale integrated (VLSI) devices, physical design is not practical without the aid of computers. As a result, most phases of physical design extensively use computer-aided design (CAD) tools, and many phases have already been partially or fully automated. Automation of the physical design process has increased the level of integration, reduced turn around time and enhanced chip performance. Several different programming languages have been created for electronic design automation (EDA), including Verilog, VHDL and TDML.
Faster performance and predictability of responses are elements of interest in circuit designs. As process technology scales to the submicron regime, interconnect delays increasingly dominate gate delays. Consequently, physical design optimization tools such as floorplanning, placement, and routing are becoming more “timing-driven” than the previous generation of tools. For such a tool to be effective, it must be able to efficiently compute interconnect delay since several million delay calculations are required to optimize a design. In certain types of circuits, delays exist based on circuit topology and circuit components. Delays are particularly acute in circuits having resistive and capacitive elements, or RC circuits. Circuit designers continually search for efficient techniques for accurate estimation of these delays, while determining the particular circuit's response to a load. In particular, circuit designers want to be able to calculate reliable delay information when designing the circuit. To this end, several prior art metrics (i.e., computational methods) have been developed.
The Elmore delay metric, which calculates the first moment of the impulse response, is the most widely applied and simplest interconnect delay metric that still captures some amount of metal resistance effects. The Elmore metric provides an upper bound on delay given any input waveform because the RC circuit impulse response is unimodal and positively skewed. The Elmore delay metric is commonly utilized for performance optimization tasks such as floorplanning, placement, buffer insertion, wire sizing in part and global routing. The widespread use of the Elmore delay metric is due to its closed form expression, fast computation speed, and fidelity with respect to simulation. Closed form delay equations, such as Elmore delay metric, are generally preferable due to both efficiency and ease of implementation, as long as they are sufficiently accurate.
Owing to the tremendous drop in VLSI feature size, a huge number of buffers (i.e., amplifiers or inverters) are needed for achieving timing objectives and fixing electrical violations for interconnects. It is estimated that the number of buffers will rise dramatically, reaching about 15% of the total cell count for intrablock communications for 65 nanometer technology, and close to 800,000 buffers required for 50 nanometer technologies. Therefore, both the complexity and importance of buffer insertion is increasing in an even faster pace.
Physical synthesis is prominent in the automated design of integrated circuits such as high performance processors and application specific integrated circuits (ASICs). Physical synthesis is the process of concurrently optimizing placement, timing, power consumption, crosstalk effects and the like in an integrated circuit design. This comprehensive approach helps to eliminate iterations between circuit analysis and place-and-route. Physical synthesis has the ability to repower gates, insert buffers, clone gates, etc., so the area of logic in the design remains fluid. However, physical synthesis can take days to complete.
During physical synthesis, buffer insertion is called for to either optimize nets for delay or to fix nets due to electrical violations. One mechanism for performing buffer insertion on a fixed Steiner integrated circuit topology is the van Ginneken algorithm, as described in “Buffer Placement in Distributed RC-tree Networks for Minimal Elmore Delay,” ISCAS Proceedings pp. 865-868 (1990). Given a fixed Steiner tree topology, the van Ginneken algorithm finds the optimal buffer placement on the topology under an Elmore delay model for a single buffer type and simple gate delay model. The primary idea of van Ginneken is to choose a set of buffer candidate locations that lie on the Steiner topology at some uniformly fixed distance apart. Buffer insertion then proceeds for that particular set of candidates from sink to source. When a candidate location falls within a region that is blocked because it is too densely populated with logic, that location is skipped and a buffer is inserted into the next candidate insertion location that is not blocked while maintaining the uniform spacing.
As seen in FIG. 1, a Steiner tree representation of a net 1 places candidate buffer insertion points 2 at regular intervals along the paths from source 3 to sinks 4, 5 and 6. A logic cell 7 (or a portion of a logic cell) is present in the path of the net. The portion of the net block by logic cell 7 is skipped when determining candidate buffer insertion points 2. The spacing between candidate insertion points may be increased or decreased by the designer to achieve a particular timing requirement. Increasing the frequency of buffer insertion locations can improve timing of the net, but at an increased buffer cost.
One problem with the van Ginneken approach is that buffer insertion fails to take into consideration more realistic net parameters, such as input signal polarities and slew constraints. From extensive experiments of traditional buffer insertion algorithms on typical industrial circuits with 300,000 or more objects, there is a big difference between the slack improvement computed by buffer insertion and the slack improvement computed by a static timing analysis tool. Traditional buffer insertion does not consider these differences and may result in suboptimal results. Van Ginneken's algorithm also restricts buffer placement to uniform locations along a net, which fails to take advantage of the density of the logic in the vicinity of the candidate locations.
In light of the foregoing, it would be desirable to devise an improved method of selecting buffer insertion locations to efficiently achieve timing closure. It would be further advantageous if the method could allow for buffer insertion with non-uniform spacings to take advantage of variations in the density of the logic regions.